1. Field of the Invention
The present invention relates to an axial plunger pump or motor in which a sealing property of a sliding surface between a valve plate and a cylinder block has been improved.
2. Description of the Prior Art
An axial plunger pump or motor which sucks and discharges a liquid or drives a rotary shaft by making plungers reciprocate in parallel to a center axis of a cylinder block has a relatively high efficiency and little pulsation as compared with the other types of pumps or motors such as a gear pump or motor, a vane pump or motor, etc. But on the other hand since the number of parts is large and the structure is complex, it is necessary to enhance the machining precision of these parts. Moreover, due to the fact that a torque moment caused by a hydraulic pressure and a tilt associated with bending of a rotary shaft would be generated in a cylinder block and hence the cylinder block would unevenly butt against a valve plate, there was a fear that these members may be seized by each other or a high-pressure liquid may leak out through a clearance therebetween.
Therefore, various axial plunger pumps or motors with a contrivance made on a seal portion between a cylinder block and a valve plate have been developed. For instance, an improved structure is known, in which, between a valve plate 103 fixed to a casing 102 which supports a rotary shaft 101 and a cylinder block 104 fixedly fitted to the rotary shaft 101, there is interposed a seal plate 106 that is always urged against the valve plate 103 by a compression coil spring 105 as shown in FIG. 1(a). Sealing between a bore 108, in which a plunger 107 is fitted slidably in the direction parallel to the rotary shaft 101, and the seal plate 106 is effected by means of seal rings 111 which are mounted around a cylinder portion 109 of the seal plate 106 to be fitted in the bore 108 via a liner 110, as shown in FIG. 1(b) which illustrates in an enlarged scale the portion in FIG. 1(a) indicated by an arrow B. In the prior art, piston rings were used as the seal rings 111.
However, since the seal plate 106 having the above-described structure takes such configuration that its cylinder portions 109 are provided the same number as that of the bores 108 drilled in the cylinder block 104 in parallel to the rotary shaft 101 as projected in an annular array, the precision in position of these cylinder portions 109 is not so high, and necessarily the gap clearance between the inner diameter of the bore 108 and the outer diameter of the cylinder portion 109 must be chosen large. Accordingly, in order to enhance a durability of the cylinder portion 109, a wall thickness at this portion is made thick, and consequently, there is a shortcoming that a cross-section area of a liquid passageway within the cylinder portion 109 is reduced and hence a self-sucking performance is degraded. In addition, due to the fact that seal rings 111 are employed, a leaking rate of a high-pressure liquid through the engaging portions of the seal rings 111 becomes an unnegligible rate, and it becomes a cause of bringing about lowering of a volume efficiency.